According to the relevant european standards, the conventional cold rolled stainless steel plate has two kinds of surfaces, one is the 2D/2B surface, the other is the surface after cold rolling, annealing, pickling and leveling. The second one is the 2R surface, which is cold rolled and annealed after smooth annealing. Before welding, clean up the paint, dirt and oxide layer of, the former is usually produced by horizontal high yield continuous annealing pickling line, and the annealing of stainless steel strip is carried out in oxidizing atmosphere. The oxidation layer generated in the annealing must be removed and then recovered to the passivation state. This process is generally achieved by electrolysis descaling and chemical pickling, and chemical acid washing is usually used as a mixed acid solution (i.e., nitric acid and hydrofluoric acid). And a large number of pollutants such as nitrogen oxides, nitrate and sludge in the waste water will be treated by special treatment process.
The latter is produced on a vertical bright annealing unit. The stainless steel plate is annealed in the mixture of hydrogen and nitrogen. If the dew point is well controlled and the oxidation of the strip surface is avoided, the pickling process is not necessary. Because of the surface features and good appearance of the surface, the stainless steel strip on the 2R surface is more popular.
By combining the main advantages of the two processes, an innovative processing technology has been developed and applied to the annealing and pickling treatment of stainless steel plate cold rolled strip.
It is a new stainless steel strip production technology that can get close to the 2R’s enhanced surface quality. It can also maintain the same production capacity and production cost as the traditional annealing pickling line, and it can also reduce the impact of chemical pickling process on the environment. Compared to the traditional annealing and pickling line, the new technology can significantly reduce the oxidation of stainless steel strip in annealing period, so chemical pickling treatment can cancel or reduce the use, in order to get better surface quality, can save the cost and reduce the need for acid pickling wastewater treatment capacity. Controlling the formation of oxide layer on the surface of stainless steel plate is through the special strip heat treatment cycle and accurately controlling the amount of oxidation in each process step (heating and cooling section), especially in the high-temperature process section.
A key step in this process is the first control section of rapid heating under oxidizing atmosphere, the oxide film and a thin layer of oxide crystal nucleus formation, followed by annealing, mechanical properties, grain size, the carbon demand of the solution and so on, this process is carried out in a non oxidizing atmosphere, can limit the generation of oxide layer the. In the cooling section of non oxidizing atmosphere, the cooling rate can prevent carbides from precipitating.
The main advantages of the new process is reflected in the production capacity and the same traditional annealing and pickling line and flexible production, saving equipment investment and management costs, reduce environmental pollution and reduce waste disposal costs, but also get better surface quality of products, the last is that this technique is suitable for the new factory is also suitable for the transformation of the existing factory.
Source: China Stainless Steel Plate Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)